A spark plug is known which includes a center electrode and a ground electrode arranged at a discharge gap away from a front end portion of the center electrode so as to generate a spark discharge between the center electrode and the ground electrode for ignition of an air-fuel mixture in a combustion chamber of an internal combustion engine.
Amid recent calls for global environmental protection, it is more strongly demanded to provide energy savings, regulate emissions of carbon dioxide and reduce emissions of unburned gases (hydrocarbon compounds). In order to satisfy these demands, developments are being actively made in internal combustion engines such as lean-burn engine, direct gasoline-injection engine and low emission gas engine. Further, exhaust gas recirculation (EGR) systems, which recirculate a part of exhaust gases into combustion chambers to reduce negative engine loads in intake strokes and produce more cleaner exhaust emissions, are being actively introduced into the lean-burn engines. Under such circumstances, it is required that the spark plug ignites a lean air-fuel mixture containing a large amount of inert exhaust gases. Spark plugs with higher ignition performance are thus needed.
One known type of spark plug with improved ignition performance includes a center electrode having a noble metal tip welded to an electrode body thereof and a ground electrode having a protruding region formed by e.g. welding a cylindrical noble metal tip, with an annular surface of the cylindrical noble metal tip directly facing the noble metal tip of the center electrode, so as to generate a spark discharge between these noble metal tips. There is proposed another type of spark plug in which a ground electrode has a protruding region formed by press forming (See Patent Document 1.)    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-286469
In the case where the protruding region is formed by welding the noble metal tip to the ground electrode, the spark plug attains improved ignition performance but has a problem of increase in manufacturing cost due to the use of the expensive noble metal tip. In the case where the protruding region of the ground electrode is formed by press forming, the press forming process causes plastic deformation in the ground electrode so that the ground electrode becomes susceptible to breakage. This results in an increased possibility that the ground electrode will break when bent to a substantially L-shaped form during the manufacturing of the spark plug or when subjected to external force during the use of the finished plug product. The spark plug thus has a problem of difficulty in securing durability.